EP1363722A1 - Pression atmospherique, decharge luminescente, source d'emission optique pour l'echantillonnage direct de substances liquides - Google Patents

Pression atmospherique, decharge luminescente, source d'emission optique pour l'echantillonnage direct de substances liquides

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Publication number
EP1363722A1
EP1363722A1 EP02705854A EP02705854A EP1363722A1 EP 1363722 A1 EP1363722 A1 EP 1363722A1 EP 02705854 A EP02705854 A EP 02705854A EP 02705854 A EP02705854 A EP 02705854A EP 1363722 A1 EP1363722 A1 EP 1363722A1
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EP
European Patent Office
Prior art keywords
capillary
discharge end
discharge
power source
electrode
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP02705854A
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German (de)
English (en)
Inventor
R. Kenneth Marcus
W. Clay Davis
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Clemson University Research Foundation (CURF)
Original Assignee
Clemson University Research Foundation (CURF)
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Filing date
Publication date
Application filed by Clemson University Research Foundation (CURF) filed Critical Clemson University Research Foundation (CURF)
Publication of EP1363722A1 publication Critical patent/EP1363722A1/fr
Withdrawn legal-status Critical Current

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/66—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light electrically excited, e.g. electroluminescence
    • G01N21/67—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light electrically excited, e.g. electroluminescence using electric arcs or discharges

Definitions

  • the present invention relates to spectrochemical sources and more particularly to glow discharge sources.
  • Glow discharge (GD) plasmas have been used as spectrochemical (i.e., optical emission) sources for well over 100 years, dating back to the early studies of atomic structure.
  • the low pressure, low power plasmas are easily controlled and yield emission spectra that are principally atomic in nature.
  • the combination of cathodic sputtering as a means of introducing atoms from bulk solids and the relatively simple optical spectra lead to the implementation of hollow cathode GD devices as line sources for atomic absorption spectrophotometry.
  • the development of the Grimm-type glow discharge geometry lead to the use of glow discharge optical emission spectroscopy (GD-OES) as a tool for both bulk solid and depth resolved analysis of metals and alloys.
  • the subsequent introduction of radio frequency (rf) powering schemes opened up the scope of application further to nonconductive materials and coatings.
  • LC-MS transport-type liquid chromatography-mass spectrometry
  • electrolyte- cathode discharge ELCAD
  • the electrolyte- containing solution was disposed in a basin having two regions separated by a glass frit.
  • a graphite rod that was electrically maintained at the cathodic potential of the discharge circuit was submerged in one of the regions of the basin.
  • a central inlet tube passed vertically through the other region of the basin.
  • the analyte-containing solution was continuously re-circulated at flow rates of 2 to 10 milliliters per minute (mL/min) through the central inlet tube so as to form a small stationary "waterfall” with a slope of about 60 degrees at the edge of the central inlet tube.
  • a tungsten electrode (acting as the anode) was mounted one to five millimeters (mm) above this slope of the waterfall.
  • the glow discharge formed in the space between the end of the anode and the slope of the waterfall.
  • the glass frit separated the region of the basin containing the waterfall from the region containing the cathode rod in order to eliminate the evolution of H 2 gas and possible explosion.
  • Detection limits for more or less bulk solutions of metal analytes produced detection limits of approximately 0.1 to 1 part per million (ppm), though for total analyte solution volumes of more than 10 milliliters (mL).
  • LOD Limits of detection
  • the present invention includes a new liquid sampling- atmospheric pressure glow discharge (LS-APGD) optical emission source for the direct analysis of metals and non-metals in electrolytic solutions.
  • a hollow capillary has an electrically conducting element that electrically communicates with the interior of the capillary and thus with the electrolyte fluid passing through the capillary.
  • the capillary has an inlet end and a discharge end disposed opposite to the inlet end.
  • the discharge end of the capillary can be formed of metal, and in other embodiments the discharge end of the capillary can be formed of material that is electrically insulating or semiconducting.
  • a mechanism is provided to move the electrolytic solution through the capillary at rates in the range of about 0.5 FL/min to about 5 mL/min at atmospheric pressure.
  • a counter-electrode is fixed to a movable stage and is disposed at a predetermined distance from the discharge end of the capillary. This predetermined distance defines an electrode gap and is typically in the range of about 0.5 to about 5 mm.
  • An electrical power source is electrically connected between the electrically conducting element of the capillary and the counter-electrode so that a potential difference can be placed between the counter-electrode and the electrically conducting element of the capillary.
  • a plasma whose i-V characteristics are within the range of conventional, abnormal glow discharges operating in the 0.1 to 10 Torr pressure regime is created.
  • the discharge end of the capillary can be surrounded by a cylinder that carries a flow of gas such as nitrogen that cools the discharge end and shapes the glow discharge.
  • An injector can be connected in fluid communication with the capillary for introducing into the electrolyte solution flowing through the capillary, fluid that contains samples of analyte, i.e., materials to be analyzed.
  • the glow discharge plasma is initiated at the surface of the solution as the solution passes from a 0.254 mm inside diameter stainless steel tube at solution flow rates of 1 to 3 mL/min.
  • the discharge shows abnormal discharge behavior, with the operating voltage being dependent on the pH/pLi/pNa of the solution and the inter-electrode gap.
  • the LS-APGD runs stably at rates of 0.5-1.5 mL min and also permits the direct introduction into the flow of the electrolyte solution, an injection of analyte in solution.
  • an injector is connected in fluid communication with the capillary so that the sample can be introduced in a flow injection mode, with sample volumes as small as about 5 FL injections.
  • Fig. 1a is a diagrammatic representation of an embodiment of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) apparatus of the present invention.
  • Fig. 1b is a diagrammatic representation of the operation of an embodiment of the liquid sampling-atmospheric pressure glow discharge (LS- APGD) apparatus of the present invention.
  • LS- APGD liquid sampling-atmospheric pressure glow discharge
  • Fig. 1c is a diagrammatic representation of an embodiment of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) apparatus of the present invention configured as an integrated detector for lab-on-a-chip applications.
  • Fig. 2a is a graphic representation of the current-voltage characteristics of an embodiment of the LS-APGD for different inter-electrode gap distances at normal polarity.
  • Solution flow rate 1.5 mUmin.
  • HNO 3 HPLC water electrolyte
  • pH 1.
  • Fig. 2b is a graphic representation of the current-voltage characteristics of an embodiment of the LS-APGD for different inter-electrode gap distances at reversed polarity.
  • Solution flow rate 1.5 mL/min.
  • Fig. 3a is a graphic representation of the effect of electrolyte solution pH on the current-voltage characteristics of an embodiment of the LS-APGD at normal polarity.
  • Solution flow rate 1.5 mL/min.
  • inter-electrode gap 1 mm.
  • Fig. 3b is a graphic representation of the effect of electrolyte solution pH on the current-voltage characteristics of an embodiment of the LS-APGD at reversed polarity.
  • Solution flow rate 1.5 mL/min.
  • inter-electrode gap 1 mm.
  • Fig. 4a is a graphic representation of the effect of Na+, Li+ and H+ concentrations on the i-V characteristics of an embodiment of the LS-APGD device at normal polarity.
  • Solution flow rate 1.5 mL min.
  • inter-electrode gap 1 mm.
  • Fig. 4b is a graphic representation of the effect of Na+, Li+ and H+ concentrations on the i-V characteristics of an embodiment of the LS-APGD device at reversed polarity.
  • Solution flow rate 1.5 mL/min.
  • inter-electrode gap 1 mm.
  • Fig. 5 is a graphic representation of the optical emission spectrum from the introduction of pH 1 deionized water electrolyte solution in an embodiment of the LS-APGD of the present invention.
  • Fig. 6 is a graphic representation of Na (I) 589.0 nm optical emission transient for the introduction of a 5 ⁇ L aliquot of a 50 ppm nA solution in an embodiment of the LS-APGD of the present invention.
  • Fig. 7 is a graphic representation of the effect of discharge current on the response of Na (I) 589.0 nm transient peak height and area in an embodiment of the LS-APGD of the present invention.
  • Fig. 8 is a graphic representation of the effect of inter-electrode gap on the Na (I) 589.0 nm transient peak height and area in an embodiment of the LS- APGD of the present invention.
  • Fig. 9 is a graphic representation of the effect of pH and pLi on the Na (I) 589.0 nm transient peak height and area in an embodiment of the LS-APGD of the present invention.
  • Fig. 10 is a chart of analytical response functions and limits of detection for an embodiment of the LS-APGD device of the present invention.
  • Fig. 11 is a diagrammatic representation of an embodiment of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) apparatus of the present invention.
  • LS-APGD liquid sampling-atmospheric pressure glow discharge
  • Fig 12 is an expanded view of a portion taken from Fig. 11.
  • Fig 13 is an expanded view with portions shown in cross section and portions shown cut away of a portion taken from Fig. 11.
  • Fig 14 is an expanded view of a portion taken from Fig. 13.
  • Fig 15 is a graphic representation of an optical emission chromatogram for the introduction of a 5 ⁇ L aliquot of a solution containing a mixture of selenoamino acids (50 ppm each), separated by reverse phase liquid chromatography prior to introduction in an embodiment of the LS-APGD of the present invention.
  • the liquid sampling-atmospheric pressure glow discharge (LS-APGD) apparatus of the present invention includes a hollow capillary 22 that has an electrically conducting element such as a metallic section 25 and an inlet end 23 and a discharge end 24 disposed opposite to the inlet end.
  • the LS-APGD includes a mechanism for moving an electrolytic solution through the capillary and past the electrically conducting element so as to be expelled from the discharge end of the capillary at rates in the range of about 0.5 FL/min to about 5 mL/min at atmospheric pressure.
  • a counter-electrode 34 is fixed to a selectively movable translation stage 37 and is selectively disposed at a predetermined distance from the discharge end 24 of the capillary. This predetermined distance defines an electrode gap that is typically in the range of about 0.5 to about 5 mm.
  • an electric power source 40 is electrically connected via electrical leads 29a, 29b between the metallic section 25 of the capillary 22 and the counter-electrode 34 so that a potential difference can be placed between the counter-electrode and the metallic section of the capillary.
  • this power source 40 can be a direct current source.
  • the power source 40 can be a radio frequency power source or a microwave frequency power source, as desired.
  • the LS-APGD device of the present invention is a glow discharge optical emission spectroscopy (GD-OES) source that operates at atmospheric pressure.
  • One of the discharge electrodes of the LS-APGD device of the present invention is formed by the electrolytic solution, which may contain one or more analyte specimens.
  • the passage of electrical current (either electrons or positive ions) across the solution/gas phase interface causes local heating and the volatilization of the analyte species.
  • the LS-APGD of the present invention totally consumes the aqueous electrolyte solutions at flow rates of up to about 2.0 mL/min, i.e., no water drips from the discharge end 24 of the capillary 22.
  • the heat generated by the glow discharge 36 vaporizes the electrolyte solution that reaches the discharge end 24 of the capillary 22.
  • a glow discharge plasma 36 whose i-V characteristics are within the range of conventional, abnormal glow discharges operating in the 0.1 to 10 Torr pressure regime is created between the emerging surface of the exiting electrified electrolyte solution 27 and the counter-electrode 34.
  • an injector 44 can be connected in fluid communication with the capillary 22 for introducing into the electrolyte solution flowing through the capillary, fluid that contains samples of materials to be analyzed.
  • a manual pump such as a syringe
  • the electrolyte solution is pumped from HPLC pump 30 and flows through the pump's output conduit 30a in the direction indicated by the arrow designated by the numeral 30b into injector 44.
  • the electrolyte solution flows into and through injector 44 and exits via output conduit 44a in the direction indicated by the arrow designated by the numeral 44b into a liquid chromatography column 31 (described below) and from there into capillary 22.
  • Manual rotation of the handle 43 of injector 44 determines whether the injector is configured and activated to introduce the predetermined sample into the stream of electrolyte solution that is flowing through the injector 44 and exiting the injector's output conduit 44a.
  • An electrical lead 41 is attached to ground the portion of the output conduit 44a through which the electrolyte solution passes before the electrolyte solution enters liquid chromatography column 31.
  • FIG. 1a An alternative embodiment of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) apparatus of the present invention is shown diagrammatically in Fig. 1a and is represented generally by the numeral 20.
  • the source 20 includes a hollow capillary 22 having an inlet end 23 and a discharge end 24 opposite the inlet end.
  • the capillary has an electrically conducting element in the form of a metallic section 25.
  • the metallic section 25 of the capillary 22 desirably is formed of stainless steel, and the inside diameter of the capillary 22 can be 0.254 mm.
  • Fig. 1a An alternative embodiment of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) apparatus of the present invention is shown diagrammatically in Fig. 1a and is represented generally by the numeral 20.
  • the source 20 includes a hollow capillary 22 having an inlet end 23 and a discharge end 24 opposite the inlet end.
  • the capillary has an electrically conducting element in the form of a metallic section 25.
  • the capillary 22 defines a longitudinal axis 26 aligned parallel to the direction of the flow of the electrolyte solution 27 through the capillary.
  • the discharge end 24 of the capillary 22 is desirably disposed such that the longitudinal axis of the capillary at the discharge end is disposed generally parallel to the horizontal, with the concomitant effect on the direction of the flow of the electrolyte solution 27 that exits the discharge end 24 of the capillary 22.
  • the discharge end 24 is a free end that desirably terminates in a plane that is perpendicular to the direction 26 of fluid flow upon exiting of the discharge end of the capillary.
  • the discharge end 24 can be formed of material that is electrically conducting (such as metal for example), material that is electrically insulating (such as glass or fused silica for example) and/or material that is electrically semiconducting (such as silicon).
  • a means can be provided for flowing gas around the discharge end of the capillary.
  • the gas flowing means desirably can include a section disposed about the discharge end of the capillary.
  • the discharge end 24 of the capillary 22 can be surrounded by a conduit such as a concentric cylinder 32.
  • This conduit 32 desirably can be connected to a supply of gas such as a canister 33 supplying nitrogen gas shown schematically in Fig. 11 for example.
  • the flow of nitrogen gas (indicated schematically in Figs.
  • the gas 38 exiting from the annular space that is defined between the exterior of the discharge end 24 of the capillary 22 and the interior of the surrounding cylinder 32 is believed to tend to confine the plasma 36 more tightly around the longitudinal axis 26 of the discharge end 24 of the capillary 22.
  • the gas 38 exiting from the annular space that is defined between the exterior of the discharge end 24 of the capillary 22 and the interior of the surrounding cylinder 32 is believed to create an environment that is especially conducive to the formation of the glow discharge 36.
  • the electrically conducting element (e.g., metallic section 25) of the capillary 22 must be electrically insulated from the rest of the apparatus that is disposed upstream from the electrically conducting element. This can be accomplished for example by providing an electrically insulating conduit that forms the portion of the capillary 22 that is disposed upstream from the electrically conducting element (e.g., metallic section 25).
  • the capillary 22 can be formed entirely of electrically insulating material and the electrically conducting element can be formed as an electrically conducting probe (such as a metal wire) that enters the interior of the capillary 22 through a side wall of the capillary.
  • the inlet end 23 of the capillary can be formed of metal or of an electrically insulating material such as a polymer like poly-ether ether ketone (a.k.a. PEEK).
  • a polymer like poly-ether ether ketone a.k.a. PEEK
  • the electrically insulating portion of the capillary 22 can be connected to the metallic section 25 by using a fitting 28, which can be composed of metal or a polymer (a.k.a. a PEEK fitting).
  • a mechanism is provided for moving an electrolyte solution through the capillary and discharging the electrolyte solution out of the discharge end 24 of the capillary at a predetermined rate in the range of about 0.5 FL/min to about 5 mL/min at atmospheric pressure.
  • This can be accomplished in a number of ways.
  • one such mechanism is a high precision liquid pumping system such as a high performance liquid chromatography (HPLC) pump 30.
  • HPLC high performance liquid chromatography
  • HPLC pump 30 is configured so that the flow of electrolytic solution that is discharged out of the discharge end 24 of the capillary 22 occurs at rates in the range of about 0.5 FL/min to about 5 mL/min at atmospheric pressure.
  • a suitable HPLC pump is a Waters (Milford, MA) Model 510 HPLC pump.
  • the discharge of the electrolyte solution is accomplished by inducing electro-osmotic flow of the electrolyte solution through the capillary 22.
  • a direct current power supply 40 places an electric potential along the length of a section of the capillary 22 that terminates in the discharge end 24 of the capillary 22. Because of this potential, an electric field causes the positively charged particles in the electrolyte solution inside the capillary 22 to migrate toward the discharge end 24. As these positively charged particles move, they carry along the non- charged species due to the effect of the fluid's viscosity, and momentum carries the solution out of the discharge end 24 of the capillary 22.
  • the flow of the electrolyte solution through the capillary 22 is accomplished by capillary action.
  • the discharge of the electrolyte solution can be aided by the flow of the gas 38 exiting from the annular space between the exterior of the discharge end 24 of the capillary 22 and the interior of the surrounding cylinder 32.
  • this exiting gas flow 38 acts to draw out the electrolyte solution 27 from the discharge end 24 of the capillary.
  • a counter- electrode 34 is disposed at a predetermined distance from the discharge end 24 of the capillary 22.
  • Fig. 14 the discharge of the electrolyte solution through the capillary 22 is accomplished by capillary action.
  • this exiting gas flow 38 acts to draw out the electrolyte solution 27 from the discharge end 24 of the capillary.
  • a counter- electrode 34 is disposed at a predetermined distance from the discharge end 24 of the capillary 22.
  • the discharge end 24 of the capillary 22 and the counter-electrode 34 form the input and output electrodes of the LS-APGD apparatus 20.
  • the predetermined distance between the discharge end 24 of the capillary 22 and the counter-electrode 34 defines an electrode gap 35 in which the plasma 36 (glow discharge) is formed.
  • a 2.4 mm diameter solid copper rod provides a suitable counter-electrode 34.
  • one or both of the input and output electrodes can be mounted on a translation stage 37 that is selectively movable so that the gap 35 (Fig. 1a) between the electrodes can be varied.
  • counter-electrode is shown aligned (at 180 degrees) with the longitudinal axis 26 of the discharge end 24 of the capillary in Fig. 1b for example, in any desired configuration where additional room is needed close to the glow discharge 36, the counter-electrode 34 can be aligned parallel but off- axis or asymmetrically intersecting the axis 26 but to the side.
  • One such desired non-axial configuration would be to accommodate the inlet of a mass spectrometer.
  • a power source 40 is connected between the capillary 22 and the counter-electrode 34 so as to place a potential difference in the range of about 200 to 1 ,000 volts between the discharge end 24 of the capillary 22 and the counter-electrode 34.
  • the output of the power source 40 can be varied by the operator.
  • the power source can be a radio frequency power source or a microwave frequency power source or a direct current power source, as desired.
  • a suitable direct current power source for maintaining the atmospheric pressure glow discharge can be provided by a Kepco (Flushing, NY) Model BHA 2000-0.1M power supply that is electrically connected to the input and output electrodes.
  • the same power supply 40 that is used to create the electro-osmotic effect needed to move the electrolyte solution through the capillary 22, can be used to provide the power needed to maintain the plasma 36 that produces the glow discharge 36 at atmospheric pressure.
  • the electrically insulating conduit that can be provided to form the inlet end 23 of the capillary 22 electrically isolates the high voltage that is applied to the metallic section 25 of the capillary 22. It also is desirable to add ballast to the discharge circuit that is formed by the metal section 25 of the capillary 22, the power source 40, the counter-electrode 34, and the plasma 36 from which the glow discharge emanates. Accordingly, as schematically shown in Fig. 1a, the high voltage that the power source 40 applies to the powered electrode is desirably coupled through a variable resistor 42 such as a 10 k ⁇ variable resistor. Similarly, in the alternative embodiment that is schematically shown in Fig.
  • a secondary electrical power source 39 can be used to ensure that an electrical potential difference exists between the discharge end 24 and the counter-electrode 34 in order to maintain the glow discharge 36.
  • the counter-electrode 34 typically is the powered (or input) electrode
  • the capillary 22 is typically the output electrode.
  • the apparatus of the present invention affords the operator the option of applying discharge voltages in each of two ways. They can be applied such that the electrolytic solution functions as the cathode of the circuit. Alternatively, they can be applied such that the electrolytic solution functions as the anode of the circuit.
  • the electrically conducting element (such as metallic section 25) of the capillary 22 can be electrically connected to the electrical power supply 40 so as to become the powered (or input) electrode while the solid copper rod 34 can become the output electrode.
  • the situation wherein the electrolyte is grounded via grounding of the electrically conducting element such as metallic section 25 of the capillary 22 (as shown in Fig. 1a for example) and a positive voltage is applied to the copper counter electrode 34 is termed the "normal" polarity mode and is shown in Figs. 11 , 1a, 1b and 1c for example.
  • the opposite configuration i.e., where the electrolyte is attached to the positive output of the power supply via attachment of the electrically conducting element (such as metallic section 25) of the capillary 22 and the copper rod 34 is grounded, is termed the "reversed" polarity mode.
  • a fluid injector 44 can be connected in fluid communication with the capillary 22.
  • the injector 44 is desirably configured for introducing into the capillary 22, fluid containing analyte, i.e., samples of materials to be analyzed.
  • a Rheodyne (Cotai, CA) Model 7725i sample injector valve provides one example of a suitable injector 44 and functions to permit the introduction of a precise and discrete volume of the sample into the flowing electrolyte solution.
  • chromatography column can be configured to effect the separation by one of the following techniques: normal phase liquid chromatography, reverse phase liquid chromatography, ion chromatography, and capillary electrochromatography. Capillary zone electrophoresis could also be employed to effect such a separation. As schematically shown in Fig.
  • a liquid chromatography column 31 can be included to define a section of the fluid pathway through which the electrolyte solution flows toward the discharge end of the capillary 22.
  • the chromatography column 31 desirably is disposed downstream of the injector 44 and upstream of the electrically conducting element (such as metallic section 25) of the capillary 22.
  • the species in the electrolyte solution and in the injected sample of analyte are segregated before being discharged out of the discharge end 24 of the capillary 22 and introduced into the plasma 36 that is maintained in the gap between the counter-electrode 34 and the discharge end 24 of the capillary 22.
  • Fig. 15 is a graphic representation of an optical emission chromatogram for the introduction of a 5 ⁇ L aliquot of a mixture of Selenoamino Acids (50 ppm each) in an embodiment of the LS-APGD of the present invention.
  • the liquid chromatograph (HPLC) column 31 is used to separate the different forms of selenium compounds so that they can be quantified individually as they exit the column 31 and flow into the plasma 36 that forms at the discharge end 24 of capillary 22.
  • HPLC liquid chromatograph
  • the LS-APGD of the present invention provides a simpler and less expensive means of performing the detection portion of the analysis than current methods like HPLC coupled to inductively coupled plasma optical emission or to mass spectrometry.
  • a light directing element 46 is desirably disposed to direct to a suitable analyzing instrument, the electromagnetic radiation from the glow discharge 36 that forms in the electrode gap.
  • a suitable light directing element 46 can include a fiber optic light guide.
  • one end of a 3 mm core diameter liquid light guide 46 available from Edmund Industrial Optics, Barington, NJ
  • the opposite end of the light guide 46 can be coupled to an instrument 50 for analyzing electromagnetic radiation that emanates from the glow discharge.
  • a suitable such analyzing instrument 50 can include a monochromator.
  • the opposite end of the light guide 46 can be coupled to the entrance slit of an optical spectrometer 50 such as a Digikrom Model 240 monochromator (CVI Laser Corp., Albuquerque, NM) 0.24m Czerny-Turner spectrometer equipped with a 2400 groove/mm holographic grating for optical analysis and monitoring of the emission from the sample.
  • the control interface of the monochromator can be used to adjust the scanning range, slit width, spectral calibration, and wavelength selection of the monochromator.
  • a photo-multiplier tube (e.g., from Hamamatsu, Bridgewater, NJ Model) can be disposed to detect the optical emission signals.
  • An analog current meter can be connected to the photo-multiplier tube and can convert the optical emission signals into voltage signals.
  • a Macintosh llsi computer can be employed to record the output of the current meter via a National Instruments (Austin, TX) NB-MIO-16X interface board.
  • An X-Y recorder-type program within the National Instruments LabView 2 software environment can be used to record the data.
  • the obtained digital data can be processed and managed in the form of Microsoft (Seattle, WA) Excel files.
  • a mass spectrometer is another instrument that can be used to analyze the constituents of the electrolyte solution, with or without any additional analytes that might be injected into the electrolyte solution flowing through the capillary 22.
  • a commercial particle beam LC mass spectrometer such as a Benchmark brand instrument available from Extrel Corporation, Pittsburgh, PA can be disposed near the plasma 36 in a conventional manner for analyzing ions that emanate from the glow discharge.
  • analyte species that are ionized through collision with electrons, excited state, or ionic species are sampled via the ion optics that extracts ionic species from the plasma 36 and directs them to a quadrupole mass analyzer for example.
  • the masses of the molecules and atoms constituting the liquid flow are analyzed by a mass spectrometer such as a quadrupole mass filter (or another type of mass analyzer) that is configured to allow ions with a given mass/charge ratio to reach a detector.
  • a mass spectrometer such as a quadrupole mass filter (or another type of mass analyzer) that is configured to allow ions with a given mass/charge ratio to reach a detector.
  • a turbo molecular pump can be employed to maintain the mass spectrometer under reduced pressure.
  • test solutions were prepared with HPLC-grade water (Fisher Scientific, Pittsburgh, PA) adjusted to the appropriate PH with concentrated nitric acid (Fisher Scientific, Pittsburgh, PA) or pLi with lithium carbonate (Alfa Aesar, Ward Hill, MA).
  • Analyte standard solutions were prepared from Specpure (Alfa Aesar, Ward Hill, MA) AAS standards (1000 Fg/mL in 5% HNO 3 ).
  • Discharge operation parameters All glow discharge sources, whether operating in reduced pressure or atmospheric pressure, operate in fairly well defined regimes of current and voltage and i-V relationships. The previous ELCAD works clearly exhibit classical abnormal GD plasma behavior.
  • the discharge voltage was seen to be dependent on the inter- electrode gap and the pH of the solution (aqueous nitric acid). As seen in Fig. 2a for example, the i-V curves for the LS-APGD device are more or less typical of GD sources. As might be expected, the required discharge voltage increases with the inter-electrode gap.
  • the operating voltages for the case of the reversed polarity arrangement are lower than for the normal case. In the low current regimes, the difference is approximately 10% lower. While at the higher end of the range the differences grow to 20-25% (as the differences in slope suggest).
  • the reason for the higher discharge voltage in the case where the electrolyte solution is the cathode must lie in the differences in the secondary electron yields of water versus copper. The majority of electrons responsible for ionization in the gas phase are generated by ion bombardment of the cathode.
  • the energy cost of this process is a key factor, much as it is in the conventional low-pressure glow discharge sources, in determining the discharge voltage that is needed for sustaining the glow discharge.
  • the maintenance potential is related to the electron binding energy in the solid. While there is no analogy for liquid water, the fact that H 2 O has a higher ionization potential than most metals would suggest that the binding energy for electrons to bulk water would require greater energy as well.
  • Fig. 3a depicts the i-V curves obtained as a function of the solution pH, as dictated by the concentration of the nitric acid electrolyte at a fixed flow rate of 1.5 mL/min and an inter-electrode gap of 1mm.
  • a large dependence on pH wherein the solutions with highest proton content require much lower operating voltages than those approaching neutral conditions.
  • the slopes of the responses depicted in
  • Fig. 3a do not depend heavily on the pH. This seems to reinforce the suggestion that the slopes of the response curves are dictated by the gas-phase plasma resistance and not the solution conductivity. In fact, the differences here suggest that electrolyte concentration affects the breakdown voltages in much the same way as the inter-electrode gap. It is also easy to see that the plasmas ignited at low pH values operate much more stably than those of low conductivity. The same general response is seen in Fig. 3b for the case of the reversed polarity experiments. Low pH values yield low discharge voltages, while higher pH values require greater voltages and operate in a less stable manner.
  • the maintenance voltage for the Li-containing solution is approximately 50 volts lower than the hydronium ion case and about 100 volts lower for the Na-containing solutions. Note the fact that the slopes of the i-V curves depicted in Fig. 4a are fairly independent of the electrolyte identity and concentration. This fact further reinforces the conclusion that the plasma resistance is related to the gas phase processes rather than to the composition of the solution.
  • the operating voltages are lower in the case of the reversed polarity operation mode.
  • the plasma resistance also appears to be lower than for the normal polarity mode. The fact that the plasma source operates at lower
  • the electrolyte or more correctly that of the solvated cation, dictates the potential that is necessary in order to produce the plasma. As suggested previously, it is the ability to release charged species from the solution surface that is fundamental to the operation of the plasma and indeed to the breakdown voltage.
  • Fig. 6 depicts a typical emission response transient for the embodiment of the present invention that is schematically depicted in Fig. 1a, specifically for a 5 ⁇ L injection of a 60 ppm Na solution (as 5% HNO 3 ) introduced in a pH 1 solution flowing at a rate of 1 mL/min.
  • the transient exhibits a fairly steep leading edge as the analyte plug enters the plasma region, though there is some signal spread over a time period of approximately 10 seconds.
  • the initial portion of the peak (on the order of 3 seconds) is believed to be a reflection of the band broadening of the analyte- containing solution, with the latter tailing being due to condensation and subsequent volatilization of analyte from the solid counter-electrode.
  • Analyte Emission Response as a Function of Discharge Conditions and Electrolyte Composition In conventional low-pressure GD sources, the role of discharge current is essentially two-fold, though not explicitly a first order relationship. First, as discharge current is increased, sample ablation rates increase to provide greater numbers of analyte atoms to the negative glow region. Second, increases in current result in greater numbers of electrons within the negative glow, and these electrons are available to excite the sputtered atoms.
  • the emission response for introduced Na analyte is related to the discharge current, both in terms of transient peak height and area. Interestingly, the relationship is not a direct proportionality but more of a quadratic one. This is not necessarily unexpected, as the role of increasing discharge current is likely two-fold here as well.
  • larger currents passing through the solution surface will result in greater heating and thus higher vaporization efficiency.
  • greater numbers of electrons in the discharge will enhance the likelihood that atoms of analyte will become excited.
  • the precision with which each of the four separate injections of analyte into the electrolyte flow is made is fairly insensitive to the discharge conditions.
  • the inter-electrode gap in the LS-APGD device of the present invention has a definite influence on the operating parameters.
  • the larger gap distances require higher maintenance voltages for a given discharge current.
  • larger gaps yield lower current densities in the plasma and greater changes of analyte diffusion outside of the excitation volume.
  • the responses for the Na (I) 589.0 nm emission depicted in Fig. 8 clearly illustrate what might be expected.
  • the sample identity will affect the rate at which analyte species enter the discharge region and thus their analytical response (i.e., emission intensity).
  • the electrolyte identity may be expected to have some effect on the analyte response that is detected by the LS- APGD device of the present invention.
  • the identity and concentration of the aqueous cation affects the i-V characteristics. Therefore, it is likely that electrolyte composition will affect that observed optical response of dissolved analyte.
  • Fig. 9 illustrates the Na (I) response to concentration for the case of hydronium and lithium ion electrolyte solutions for both the case of analyte transient peak height and analyte transient peak area.
  • the response curves parallel each other for the two measurement modes, with the signals generated in the hydronium ion case being generally 10-15% higher than the signals generated for the Li-based solutions.
  • the plasma was operated in a constant current mode at a value of 40 milliamperes (mA), employing a 5 ⁇ L injection of 1000 ppm Na solution into an electrolyte flow rate of 1 mL/min.
  • Analytical Characteristics Having developed some basis of understanding of the operation of the LS-APGD device, preliminary figures of merit were established for a range of analyte species.
  • Analytical response curves were generated for the analytes Na, Fe, and Pb, at the respective atomic transition wavelengths of 589.0, 248.3, and 405.8 nm.
  • the respective analytical transitions were chosen from flame-AES tabulations.
  • Fig. 10 summarizes the analytical response curve functions that were obtained from linear least-squares fitting of emission transient peak heights and areas based on introduction of standard solutions from 50 to 1000 ppm.
  • the electrolyte solution composition was HPLC grade water adjusted to a pH of one (1) with concentrated nitric acid, and the discharge conditions were 40 mA with a one (1) millimeter inter-electrode gap.
  • Each data point in the calibration curves was established by taking the average value of peak height or area for four identical injections of the test solutions. In all instances, the sample-to-sample variability was less than 14% RSD, with the values being typically less than 5% RSD.
  • the correlation coefficients for the respective curves are routinely better for the case of peak height than for area. This reflects the variability in the tailing exhibited in Fig. 6 on a sample-to-sample basis.

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Abstract

L'invention concerne une source de spectroscopie d'émission optique de décharge luminescente (GD-OES) qui fonctionne à la pression atmosphérique. Une des électrodes de décharge du dispositif est constituée d'une solution électrolytique (27) contenant l'échantillon de substance à analyser. Le courant électrique (d'électrons ou d'ions positifs) qui traverse l'interface solution/phase gazeuse, entraîne un réchauffement local et la volatilisation des espèces de substances à analyser. Les collisions dans la zone de décharge juste au-dessus de la surface de la solution engendrent une émission optique qui est caractéristique des éléments de la substance à analyser. Le dispositif utilise la solution de substance à analyser comme cathode ou anode. Les paramètres de fonctionnement dépendent de la concentration d'électrolyte (notamment de la conduction de la solution) et l'espace (35) situé entre la surface de la solution et la contre-électrode (34). Selon des conditions classiques, les courants de décharge sont de l'ordre de 30 à 60 mA et les potentiels sont de l'ordre de 200 à 1000 volts. Une solution d'électrolyte aux valeurs de pH, pNa ou pLi de l'ordre de 0.5 à 2 et des espaces inter-électrodes de l'ordre de 0.5 à 3 mm permettent d'obtenir des plasmas stables (36), les solutions de substance à analyser étant entièrement consommées à des vitesses d'écoulement maximales d'environ 2.0 mL/min.
EP02705854A 2001-01-29 2002-01-17 Pression atmospherique, decharge luminescente, source d'emission optique pour l'echantillonnage direct de substances liquides Withdrawn EP1363722A1 (fr)

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US5086226A (en) * 1989-05-31 1992-02-04 Clemson University Device for radio frequency powered glow discharge spectrometry with external sample mount geometry
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